1. Field of the Invention
The present invention relates to a power switching device.
2. Related Background Art
MIS (Metal Insulated Semiconductor) transistors, such as power MOSFETs and IGBTs (Insulated Gate Bipolar Transistors) are used in various electronic apparatuses such as power supplies and inverters.
FIG. 9 is a circuit diagram of a conventional DC-DC converter. A DC-DC converter 500 includes a power MOSFET Q1 (here after also referred to as transistor Q1) connected between an input IN and an output OUT. The transistor Q1 is driven by a driver DR1, which is controlled at a high frequency by a control circuit IC1.
The DC-DC converter 500 further includes an inductor L, a capacitor Cin and a capacitor Cout The inductor L, the capacitor Cin and the capacitor Cout convert an input voltage Vin to an output voltage Vout by switching on or off the transistor Q1.
The DC-DC converter 500 further includes a diode DI and a power MOSFET Q2 (hereafter also referred to as transistor Q2). The diode DI and the transistor Q2 complement an output current of the DC-DC converter when the transistor Q1 is switched from ON to OFF. When the transistor Q1 is ON, therefore, the transistor Q2 is OFF. When the transistor Q1 is switched from ON to OFF, the transistor Q2 is switched from OFF to ON. In other words, the DC-DC converter 500 is a DC-DC converter of synchronous commutation type. The transistor Q2 is driven by a driver DR2, which is controlled by a control circuit IC1.
In the conventional transistor Q1, all cells are driven by using one gate electrode. The transistor Q1 has a large number of cells connected in parallel to each other in order to let a large current flow from the input to the output. Gate electrodes are provided respectively on these cells, and aluminum wiring is connected to gate electrodes. The aluminum wiring is connected to a bonding pad (not illustrated) By applying a voltage to the aluminum wiring via the bonding pad, the potential at the gate electrodes of all cells is changed. As a result, all cells are switched on or off. This means that the area of an activated cell region (hereafter referred to as activated region) depends upon the chip size and it is fixed.
For increasing the switching speed of the transistor Q1, it is effective to decrease the resistance or inductance of the aluminum wiring. In the conventional technique, the width of aluminum wiring is made wider in fabrication, or a plurality of pieces of aluminum wiring have been fabricated, in order to reduce the resistance or inductance of aluminum wiring.
In the case where the resistance of the aluminum wiring is reduced, however, the driver DR1 must let flow a large current. Therefore, the burden imposed on the driver DR1 increases. In addition, in this case, a large current flows through the bonding wire to the gate electrode. Therefore, it becomes necessary to consider the resistance and inductance of the bonding wire.
Therefore, a power switching device capable of conducting switching at a high speed is desired.